[DWARFLinker][NFC] Remove RangesTy &getValidAddressRanges().

This patch simplifies line table generation. It removes global
array of all units ranges(RangesTy &getValidAddressRanges()).
The comment says that global array of all units ranges is necessary
to handle corner cases inside line table rows. Removing that
special handling shows that its current usage is handling of
"end of range case" which is already handled correctly
(without special handling). .debug_line tables for clang binary
built with and without this patch are equal.

Differential Revision: https://reviews.llvm.org/D154288